MdePkg: Add a structure definition for the GHCB

MdePkg: Add a structure definition for the GHCB

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

The GHCB is used by an SEV-ES guest for communicating between the guest
and the hypervisor. Create the GHCB definition as defined by the GHCB
protocol definition.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>